A clinic administrator taking down Knoxville’s information shakes her head in disbelief.

Within the hour, the co-creator and breakout star of MTV’s cultishly beloved series “Jackass” – and a trio of spinoff movies that have a combined gross of more than $335 million – will be under general anesthesia and no longer able to rectify any self-inflicted finger fiascoes.

Looking jaunty in the waiting room, wearing a garrison cap and Buddy Holly glasses, Knoxville can’t quite stifle a laugh. “That would’ve been pretty silly,” he says, attempting to make a fist without the cooperation of his unbending digit.

Given the Tennessee native’s unique occupational hazards as Hollywood’s preeminent stunt doofus – a fall guy who has voluntarily been shot by a riot-control sandbag gun, offered up his left pectoral to be bitten by a baby alligator and launched himself skyward on a giant rocket Wile E. Coyote style all in the name of fun – you can forgive Knoxville for being somewhat cavalier about his physical well-being. This is the guy for whom the opening line from Roger Alan Wade’s famous country ditty “If you’re gonna be dumb, you gotta be tough” applies as both lifestyle choice and mission statement.

While filming his ribald candid-camera comedy “Jackass Presents: Bad Grandpa” (arriving in theaters Friday and featuring Knoxville, 42, as a libidinous, pratfall-prone octogenarian) the performer sustained a torn shoulder, a fractured elbow and “a screwy thing” with his foot.

But the ripped tendon that brought Knoxville to Kerlan-Jobe, primarily known for treating players from the L.A. Lakers, Dodgers, Angels and Kings, is a different kind of sports injury. One that can be traced back to the loved-up high accompanying a certain designer drug.

“We were shooting a promo at this frat house in Arizona,” Knoxville explains in the pre-op waiting area. “I was sitting with all the students watching the movie. One of them dosed me with Ecstasy. So I was running around … out of my mind. ‘No pain!’ I smashed through a table. I was climbing up a basketball net and hanging close to the rim. That may have snapped it.”

Life has a funny way of battering Knoxville over the head for giggles while also – incidentally but assuredly – helping propel his stardom. In 1998, the then-underemployed actor (ne: Philip John “PJ” Clapp) parlayed stunt demonstrations for the skateboard-humor magazine Big Brother into the “Jackass” franchise. Among the stunts, Knoxville shot himself with a .38-caliber pistol while wearing the cheapest bulletproof vest on the market and got zapped with a 120,000-volt Taser.

Forming a loose confederacy of like-minded dunces with such series regulars as Ryan Dunn, Bam Margera, Stephen “Steve-O” Glover, Jason “Wee Man” Acuna and Chris Pontius – and Academy Award-nominated writer-director Spike Jonze rounding out the team as an occasional on-camera performer and producer – the “Jackass” collective proceeded to generate shock entertainment from ritual humiliation and deliberate self-harm with a dash of winking homoeroticism thrown in for good measure.

The MTV series ran from 2000 to 2001, spawning a trio of low-budget / high-yield films in its wake (the most recent, 2010’s “Jackass 3-D,” was shot for $20 million and grossed a robust $171 million worldwide).

And Hollywood came calling to offer Knoxville minor acting gigs in such mainstream films as “The Dukes of Hazzard,” “Charlie’s Angels: Full Throttle” and “The Last Stand” opposite Arnold Schwarzenegger.

But “Bad Grandpa” represents a departure for the self-described “half-assed stuntman,” a kind of career shot in the arm.

As far back as 2006, Paramount Pictures, the studio that has theatrically distributed every “Jackass” film, began urging Knoxville to spin off a film centered on Irving Zisman, or “old man” as he’s informally known. The idea was to practical joke his way across the country a la Sacha Baron Cohen’s Borat character.

But Knoxville resisted at every turn, convinced the hidden-camera prank premise was too thin to support a traditional narrative and cowed by the rough time he was going through personally.

“I couldn’t have done this movie then,” he says. “I was really all over the place, spinning my wheels. I was out in the bars all the time. I wasn’t in the right head space. I couldn’t have written or performed it.”

Added Jeff Tremaine, producer-director of the TV series and all the “Jackass”-branded films: “From a production standpoint, it’s a terrible idea for a movie. On the show, we could wing it. We’d run around and shoot stupid stuff – whatever we could get. Never did it seem like it could sustain a story.”

Around 2008, however, the brain trust at Knoxville and Tremaine’s Dickhouse Productions arrived at a novel conceit on which to hang the “Bad Grandpa” plot line: a travelogue loosely modeled on the 1973 movie “Paper Moon,” starring father-daughter tandem Ryan and Tatum O’Neal on a cross-country odyssey as Ryan’s con-man character attempts to deliver a 9-year-old girl to relatives.

In “Bad Grandpa,” Knoxville’s old man, newly emancipated by the death of his wife, Ellie (a synthetic molding of Oscar-nominated actress Catherine Keener), spews crude but cryptic come-ons to every female within earshot. “I may be too old to stir the gravy,” Irving’s fond of saying. “But I can still lick the spoon.”

He’s the type of unreconstructed old salt to post up on a park bench and share a six-pack of beer with his 8-year-old grandson, Billy. A dirty old man given to public bouts of explosive farting, he’s the kind of guy who accidentally – excruciatingly – gets his “jim dog” stuck in the coin slot of a soda vending machine.

But when Irving’s daughter is jailed on crack-cocaine charges, the scabrous senior is reluctantly compelled to drive Billy (Jackson Nicoll) from Columbus, Ohio, to Raleigh, N.C., into the custody of the boy’s deadbeat dad.

Their grandpa-grandson camaraderie evolves thanks to some “Jackass”-flavored bonding exercises: Irving pitching through a plate-glass window on a coin-operated children’s ride and the old man entering Billy – in drag – into a beauty pageant for preteen girls.

According to those who have seen pre-release audience surveys, “Bad Grandpa” is on track for a strong opening and could take in more than its budget – as much as $35 million – in its first three days.

“When I was doing those bits for Big Brother, I always had hopes. But you can’t imagine this type of thing happening from such humble, idiotic beginnings,” says Knoxville.”

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